draw: fix vs/fs input/output mismatches

draw: fix vs/fs input/output mismatches

When we've changed draw_find_shader_output to return -1 instead
of 0 on non found attribs we broke the default behavior of
draw, which was to always redirect those to the first (0th) slot.
To preserve that behavior if draw_emit_vertex_attr notices a
mismatched vertex attrib, it just redirects it to the first slot
(instead of trying to use negative index in an array).

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>